In the past, for electron-emitting elements, the “thermal emission type”, in which a high voltage is applied to a material such as tungsten that has been heated to a high temperature, had been predominant. However, in recent years, research and development for so-called “cold cathode type” electron-emitting elements has been actively pursued. Against this background, rod-like fine particles such as carbon nanotube and carbon fiber have received attention as the component material (cold cathode member) of a highly efficient electron emission source in the cold cathode type (field emission electron emitter) because of their high aspect ratio and small radius of curvature of the tip.
For example, it has been reported before that bundled carbon nanotubes are capable of providing an emission current density of as high as 400 μA/cm2 with a turn-on voltage of as low as 64 V. It should be noted, however, that in order to use carbon nanotube as a highly efficient cold cathode member, it is required to provide electrical continuity between the electrode and the carbon nanotube by contacting the carbon nanotube with the electrode surface and to make the carbon nanotube stand substantially perpendicularly to the electrode surface. However, no techniques have been reported which allow carbon nanotube to stand substantially perpendicularly over a large area with good productivity.
De Heer et al. discloses a technique in Science, vol. 270, p. 1179 (1995) in which a suspension of carbon nanotube is passed through a ceramic filter so that the carbon nanotube is stuck in microscopic holes in the surface of the filter, and subsequently, the filter is pressed on a plastic sheet, thereby transferring the carbon nanotube on the plastic sheet. It has been reported that this technique made it possible to form a two-dimensional array of the carbon nanotube standing substantially perpendicularly to the plastic sheet and to obtain field emission of electrons from the tip of the tube.
However, in this technique, the continuity between the carbon nanotube and the electrode is hindered by the plastic sheet, and thus the operating voltage for electron emission increases. In addition, a ceramic filter with a large area is difficult to obtain, making it difficult to obtain an electron source that is patterned on a large area.
Japanese Unexamined Patent Publication No. 10-149760 discloses a technique that utilizes carbon nanotube or fullerene as an electron emitter material in a field emission cold cathode device. Specifically, a plurality of electron emitters are configured utilizing a plurality of carbon nanotubes that are arranged on the supporting substrate like fallen trees lying on top of one another. The carbon nanotubes in this case are formed, for example, in a manner such that the carbon of the anode electrode is sublimated by arc discharge and the resultant substance is then precipitated on the cathode. The precipitated carbon nanotubes are grouped together and then disposed on the electrode by a method such as coating.
However, with this technique, the carbon nanotubes tend to lie substantially parallel to the supporting substrate, which makes it difficult to arrange the carbon nanotubes so as to be substantially perpendicular to the supporting substrate. It should be noted that fullerenes are very small and thus cannot support the carbon nanotubes and make them stand. In other words, with this technique, the positions of the carbon nanotubes cannot be controlled sufficiently, and therefore it is difficult to obtain a cold cathode member having highly efficient electron-emission characteristics.
Moreover, Japanese Unexamined Patent Publication No. 10-12124 discloses a configuration of an electron-emitting element that utilizes carbon nanotube as an electron emitter. An electron emitter with this configuration is such that carbon nanotube is grown, by catalysis of a metal catalyst, in pores which are regularly arranged in an anode oxide film. This configuration and its fabrication method, however, require a great amount of time for the formation process of carbon nanotube, and therefore are not considered to provide satisfactory productivity and thus are not practical.
Furthermore, since both of the device configurations disclosed in the above-mentioned Japanese Unexamined Patent Publication Nos. 10-149760 and 10-12124 are weak in that only one end of each carbon nanotube is in contact with the electrode supporting substrate, the interaction between the supporting substrate and electrons is unstable, causing a problem of unstable operating current.
Meanwhile, in order to solve these problems, a technique is proposed in which carbon nanotube is stuck, by electrophoresis, in an organic polysilane film having been irradiated with ultraviolet (UV) rays, thereby making the carbon nanotube stand in a direction vertical to the supporting substrate (Reference: Nakayama et al., proceedings of Pan-Pacific Imaging Conference/Japan Hardcopy '98, p.p 313-316, sponsored by the Imaging Society of Japan, Tokyo, July, 1998). This technique is described with reference to FIG. 11.
First, a liquid such that carbon nanotube 1101 is dispersed in isopropyl alcohol is poured into an electrophoresis chamber 1104. Next, an electric field is applied, using an external power supply 1107, between a flat plate electrode 1105 in opposition to a supporting substrate 1103 and conductive layers 1106 patterned on the supporting substrate 1103, whereby the carbon nanotube 1101 is aligned in the direction of the electric field and at the same time is transferred, by electrophoresis, onto the conductive layers 1106. It should be noted that prior to electrophoresis, only organic polysilane films 1102 on the conductive layers 1106 are subjected to ultraviolet irradiation. By doing this, the Si—Si bonds of the organic polysilane films of the irradiated portions are broken and the films become porous, allowing the carbon nanotube 1101 to be selectively stuck in the porous portions. That is, with this technique, the carbon nanotube 1101 can be selectively arranged on the organic polysilane 1102 provided on the conductive layers 1106, by the electric field during electrophoresis, and can stand substantially perpendicularly. In addition, by patterning the organic polysilane 1102 with ultraviolet rays, a region where the carbon nanotube 1101 is to be arranged can be freely set.